Polyoxymethylene polymer stabilization



United States Patent 3,327,023 POLYOXYMETHYLENE POLYMER STABILIZATION Peter Otto Schoenholzer, Monchaltorf, Switzerland, as-

signor to E. I. du Pont de Nemours and Company,

Wilmington, Del., a corporation of Delaware N0 Drawing. Filed Sept. 11, 1963, Ser. No. 308,080 3 Claims. (Cl. 260-901) This invention deals with the stabilization of acetal resins, and, more particularly,.with the employment of a tertiary amine-containing polymer which acts as a catalyst for the stabilization of polyoxymethylene, and provides additional thermal protection after completion of the stabilization reaction.

The prior art has recognized that certain compounds containing tertiary amines are useful as catalysts in the esterification of the terminal portions of the polyoxymethylene chains with a carboxylic acid anhydride. Although these tertiary amines are efiicient catalysts for the esterification of polyoxymethylene, they have a deleterious effect on the thermal stability of the resultant product, and must be removed therefrom after completion of the esterification reaction. Cf. United States Patent 2,998,409, issued on August 29, 1961, to S. Dal Nogare et al., wherein selected tertiary amines are disclosed as catalysts for the esterification of polyoxymethylene by an acid anhydride. The relatively volatile tertiary amines are removed from the polymer during the esterification reaction and consequently impart no thermal stability to the esterified material in addition to complicating the process for recovery of the excess esterification reagent. The non-volatile amines must also be removed from the polymer in various manners because they destabilize the esterified polymer. The prior art has also been concerned with the polymerization of formaldehyde in the presence of a tertiaryamino nitrogen-containing polymers and copolymers, wherein the formaldehyde polymerizes at the site of the tertiar amino nitrogen, thereby chemically incorporating the nitrogen in the oxymethylene chain. However, the employment of such a copolymer as an esterification catalyst and thermal stabilizing agent is not disclosed nor suggested. Cf. United States Patent 2,844,561, issued July 22, 195 8, to M. F. Bechtold et al.

It is an object of the present invention to provide a polyoxymethylene carboxylate of improved thermal stability. Another object of the present invention is to provide a highly eflicient catalyst for the esterification of polyoxymethylenes. A further object of the present invention is to provide a polymeric composition which readily reacts with a carboxylic acid anhydride to form a polyoxymethylene carboxylate which exhibits exceptional thermal stability without additional treatment. Other objects will appear hereinafter.

The above objects are accomplished by reacting a polymeric composition which consists ofa high molecular weight polyoxymethylene having from 0.01 to 4%, and preferably 0.1 to 2.0%, by weight based upon the polyoxymethylene of a tertiaryamino nitrogen-containing polymer which is substantially non-volatile with a carboxylic acid anhydride, and thereafter recovering a thermally stabilized polyoxymethylene carboxylate. More particularly, the tertiaryamino nitrogen-containing polymers may have repeating units of the general formulas (CHE-CR1) 1'1, la I 1 3,327,023 Patented June 20, 1967 wherein R is selected from the class consisting of hydrogen, alkyl groups having 1 to 6 carbon atoms; R is selected from the class consisting of oxygen, carboxyl, and phenyl; R is an alkylene group having 1 to 6 carbon atoms; R, and R are alkyl groups having 1 to 6 carbon atoms wherein the total carbon atoms in groups 4 and 5 are less than 8 and n is a positive integer greater than about 10; (2) [-Rti T 1 i i] Rio where R and R are alkylene groups having 0 to 6 carbon atoms; R is selected from the class consisting of a group having the formula t R OO, O-, and -N, R is an alkylene group having 0 to 6 carbon atoms; R is an alkyl group having 1 to 12 carbon atoms; and n is a positive integer; and n-CIH:Ri2R1:iR

1'11! Rm R11 wherein R and R are alkylene groups having 1 to 6 carbon atoms; R is a group selected from the class consisting of t 9 CO-, O, and (-N, R and R are alkylene groups having 0 to 6 carbon atoms; and R and R are alkyl groups having 1 to 6 carbon atoms wherein the total carbon atoms in groups R and R are less than 8 and n is a positive integer.

The aforementioned units in Group 1 may be present as the sole constituent of the polymer chain or may be interspersed with one or more comonomers, e.g., the referred class of tertiaryamino nitrogen-containing polymers are the acrylic polymers wherein groups having the formula wherein R is selected from the class consisting of hydrogen and alkyl groups having 1 to 4 carbon atoms; R is selected from the class consisting of hydrogen, normal alkyl groups having 1 to 4 carbon atoms; aryl and substituted aryl groups having 6 to 12 carbon atoms; carboalkoxy groups having 1 to 5 carbon atoms and n is a positive integer.

In the preferred embodiment of the present invention, the tertiary amine groups are introduced in a copolymer containing 5 to by weight of the tertiaryamino nitrogen-containing acrylic polymer and 95% to 5% by weight of an alkyl methacrylate, e.g., methyl methacrylate with the preferred composition being the mixture containing 80% by weight methyl methacrylate and 20% by weight diethylaminoethyl methacrylate, In the preferred copolymer, the molecular Weight should be 1000 to 100,000 corresponding approximately to an inherent viscosity as measured in a solution of the polymer at a concentration of 0.5000 gram of polymer per ml. of chloroform at a temperature of 25 C. of 0.1 to 1.4. The vapor pressure of the copolymer should be below 1 mm. of mercury at 200 C.

The advantages of the present invention lie in the fact that compositions consisting of a high molecular weight polyoxymethylene and a tertiaryamino nitrogencontaining copolymer are readily susceptible to reaction with an esterification agent, e.g., acetic anhydride, and

after reaction with the anhydride, the amine-containing polymer may be retained in the composition for enhanced thermal stability of the final product.

The thermal stability of the polymeric compositions of the present invention was determined by heating a sample of the stabilized polymer in a glass syringe in an inert nitrogen atmosphere at the temperatures indicated and measuring the gas evolved over a 30 minute period. More particularly, the thermal stability was measured by compressing a 0.5:001 gm. sample of polymer into a pellet about mm. in diameter which is weighed to within 0.1 mg. The pellet is placed in a clean glass syringe having a capacity of 50 cc. which is sealed at the usual outlet and which has a glass tube inserted lengthwise through the plunger. This tube permits air to be expelled as the plunger is lowered and allows nitrogen to be introduced into the area of the syringe containing the polymer sample. A rubber serum stopper is placed on the outer end of the tube and the syringe is filled with nitrogen by inserting a hypodermic needle through the stopper following which the syringe is evacuated. This procedure is repeated until all the air is removed from the syringe while the plunger is coated with a small amount of silicone oil to ensure a tight fit with the body of the syringe and to provide adequate lubrication between the moving parts. After the final evacuation, the syringe is immersed in a vapor bath at the temperature indicated and the volume of gas evolved per unit time is measured. The volume of gas, in cc., evolved during a period of 30 minutes per 0.5 gram of sample is recorded as the thermal stability of the sample.

The following examples are presented to illustrate and not to restrict the present invention. Parts and percentages are by weight unless otherwise noted.

EXAMPLES 18 The apparatus which was employed to treat the composition of the present invention consisted of two concentric glass tubes, the LD. of the inside tube being 25 mm. and the ID. of the outside tube being 38 mm. with an overall length of the two tubes of 700 mm. The

cated. The esterification reagent which in these examples was acetic anhydride was introduced in the bottom of the inner tube at a preset rate, and was vaporized by the heated jacket and raised to the temperature of the vap rs in the jacket. The apparatus was operated as follows:

In each example, grams of unstabilized polyoxymethylene which had been prepared according to the general process of United States Patent 2,994,687, issued Aug. 1, 1961, to Goodman et 211. were mixed with the tertiaryamino nitrogen-containing acrylic copolymers in the amounts indicated and were charged to the reactor resting upon the aforementioned screen. Vapors of acetic anhydride were introduced at the base of the reactor passing up-flow through the polymer at a rate of 60 ml. per hour and at a temperature and for the time indicated in Table 1. In each example, after the indicated time had elapsed, the anhydride flow was stopped and the reactor was purged with nitrogen for five minutes to remove any residual acetic anhydride from the polymer. The polymer was removed from the reactor, weighed and analyzed for thermal stability which values are reported in Table 1. Examples 1 and 2 demonstrate the marked increase in thermal stability by the employment of the tertiaryamino nitrogen-containing acrylic polymer, and Examples 3 through 8 show that for each particular time of stabilization the amino nitrogen-containing polymers provided a product which had a 2- to 3-fold improvement in thermal stability as compared to polymer which had been treated with acetic anhydride in the absence of a tertiaryamino nitrogen-containing polymer.

EXAMPLES 9-16 A series of stabilization reactions were carried out according to the process described in Examples 1-8, wherein the percentage of dimethylaminoethyl methacrylate in the methyl methacrylate copolymer was varied from 11 to 100% thereby demonstrating that a wide range of the tertiaryamino nitrogen-containing acrylic polymer in the copolymer is operable as a stabilization catalyst and thermal stabilizer for the esterified resin. The results are reported in Table I.

TABLE 1 Composition of copolymer, Stabilized Thermal stability of Example Concentration of amine percent dimethyl- Time of Temperature of polymer recovered polymer No. containing copolymer aminoethyl methacrylate] stabilization stabilization recovered (percent by weight) percent methyl (minutes) 0.) (grams) methacrylate 222 0. 259 C.

outside tube was arranged as a acket for the inner tube EXAMPLE 17 and designed so that vapors of a condensable liquid could be introduced near the bottom and removed at the top. By controlling the pressure within the jacket, the temperature of the "boiling liquid could be controlled thereby controlling the temperature within the inner tube. Suitable condensers were arranged at the upper end of the reactor to handle the gas passed therethrough. Approximately 100 mm. above the bottom of the tube, a fine mesh wire screen was supported in the inner tube to retain polymer which was introduced as a finely divided powder after the tertiaryamino nitrogen-containing The stabilization reaction was carried out as described in Example 2 except that a copolymer containing 25.5% by weight diethylaminoethyl methacrylate and 74.5% by weight methyl methacrylate was added to the polymer at a concentration of 0.5% by weight prior to the stabilization rate reaction. After treatment for 60 minutes at 159 C. with acetic anhydride vapor and purging with nitrogen as described in Example 2, the polymer was removed from the reactor and found to have a thermal stability of 23.5 at 259% C. as compared to a value of for a similar polymer which had been stabilized without the polymer was mixed therethrough in the amounts indi- 75 addition of tertiaryamino nitrogen-containing copolymer.

EXAMPLE 18 The stabilization reaction was carried out as described in Example 2 except that a copolymer containing 23% by weight dimethylaminoethyl methacrylate and 77% The foregoing examples show the use of acetic anhydride as the esterification reagent, however, broadclasses of carboxylic acid anhydrides, such as those disclosed in United States Patent 2,964,500, issued Dec. 13, 1960, to

by weight styrene was added to the Polymer at a con- 5 rg ggg et are considered operable m the pres centration of 0.5% by weight prior to the stabilizatioln with the exception of Examples 17 and 18 hereim reacnon In Place of the copcilymer i m above, the acrylic copolymers illustrated hereinabove were treatment for 60 i i at.159 g -g dimethylaminoethyl methacrylate and methyl methacryimhy'dnde Vapor and Purging wlth nitrogen as escn e late. It should be noted, however, that a wide variety of Example the Polymer was refiwved from the {eactor tertiaryamino nitrogen-containing acrylic polymers from and found to have a thermal stabllity of 47 at 259 C. as acr lic monomers Such as compared to a value of 92 for a similar polymer which y had been stabilized without the addition of tertiaryamino dimethylarninoethyl acrylate, nitrogen-containing copolymer. diethylaminoethyl acrylate,

dimethylaminomethyl acrylate, EXAMPLE 19 dimethylaminomet-hyl methacrylate, The stabilization reaction was carried out as described 22:2;Egiggggfiggg gg fggfi s and in Example 2 except that a polyamide of N-dodecyllmmoth 1h 1 th 1 6th lat diacetic acid and pentamethylene diamine of a molecular e y gf y m acry' weight of about 1000 was added to the polymer at a conare a so opera centration of 1.0% by weight in place of the copolymer The foregoing tertiaryamino nitrogen-containing acrylic employed in Example 2. After treatment for 60 minutes monomers may be copolymerized with monomers such as at 159 C. with acetic anhydride and nitrogen purging methyl acrylate, methyl methacrylate, ethyl acrylate, butyl exactly as in Example 2, the polymer was found to have 25 acrylate, or, in general terms, alkyl acrylates, ethylene, a thermal stability of 6.8 measured at 222 C. as comstyrene, propylene, butadiene, and vinyl chloride. In adpared to 24.3 for a similar polymer stabilized without dition to acrylic polymer, polyamides, polyesters, such as the addition of tertiaryamino nitrogen-containing polymer. the condensation products of N-dodecyliminodiacetic acid with 1,4 butanediol and N pentyliminodiethanol with EXAMPLE 20 adipic acid, and vinyl polymers which contain a tertiary The stabilization reaction was carried out as described nitrogen atom are also-operable in the present invention. in Example 2 except that a polyester of N-dodecyliminodi- The a tive amine group may be present in the backbone acetic acid and 1,4-butanediol was added to the polymer of the polymer chain, or in a branch, depending upon at a concentration of 0.2% by weight in place of the the particular type of polymer selected. As a general rule, copolymer employed in Example 2. After treatment for the polymer employed must not attack or destabilize poly- 60 minutes at 159 C. with acetic anhydride and nitrogen oxymethylene and should be soluble in acetic anhydride purging exactly as in Example 2, the polymer was found at 140 C. and nonvolatile at 160 C. while remaining to have a thermal stability of 8.7 measured at 222 C. stable at the latter temperature. Polymers in which the as compared to 22.2 for a similar polymer stabilized nitrogen atom is adjacent to large alkyl groups and without the addition of tertiaryamino nitrogen-containpresumably sterically hindered are not suitable for the ing polymer. practice of the present invention, e.g., copolymers of EXAMPLES 21-23 dimethylaminoethyl methacrylate with lauryl methacrylate are inactive as catalysts in the present invention. The A series of stabilization experiments were carried out dj groups h ld h 1 h 8 d f bly in the apparatus as described in Example 1 at p less than 6 carbon atoms to prevent the nitrogen atom tures of and UPOII P y y y from becoming sterically hindered. The term polyoxyas described in Example 1 containing 0.5 y eight of methylene as employed herein refers to any type of acetal a copolymer C0f1tainiI1g13-3% dimet'hylaminoethyl mfithhomopolymer and copolymer which is susceptible to aclylate and 867% y Weight methyl methacrylate- In esterfication by treatment with a carboxylic acid anhyeach case, the acrylic copolymer was ad to the u dride or equivalent material, and generally will encom- Stabililed P y y Coating the Solid p y y y pass polyoxymethylenes having a terminal hydroxyl group with a solution of the copolymers in toluene. A substanwhich hydroxyl group reacts with the esterification agent tially similar series of experiments were executed at the to produce an ester end group, which end group is more same temperatures as mentioned above, but in the absence stable than the original hydroxyl group. of the tertiaryamine-c-ontaining copolymer. After the The stabilized composition of the present invention can polyoxymethylene had been treated with acetic anhydride be transformed into many useful articles by extrusion or in the absence of catalyst, a copolymer of the same cominjection molding, e.g., films, funicular structures such as position and at the same concentration as above was added filaments, bristles, and fibers, and molded parts in general. to the stabilized material; the results of these experiments In additio th composition of thi i ti may are set forth in Table II which show that the copolymer 6O tain pigments, fillers, reinforcing agents, or other polyimparts some thermal stability to the polyoxymethylene meric substances in addition to the tertiary-amino nitroeven if it is added after the stabilization reaction, but imgen-containing polymers. parts a markedly greater stabilizing effect if it is added I claim: before treatment of the polymer with acetic anhydride. 1. A process for the stabilization of high molecular TABLE II Thermal stability Concentration Example of acrylic Temperature of copolymer stabilization No copolymer Oopolymer Copolymer added added before added after stabilization stabilization weight polyoxymethylene having 1 to 2 of its terminal valences satisfied with a hydroxyl group, which process comprises adding a tertiaryamino nitrogen-containing copolymer having a molecular weight of at least 1000 to said polyoxymethylene, contacting the mixture thus obtained with a carboxylic acid anhydride, and thereafter recovering a thermally stable polyoxyrnethylene carboxylate having said copolyme-r dispersed therein.

2. A process for the stabilization of high molecular Weight polyoxymethylene having 1 to 2 of its terminal valences satisfied with a hydroxyl group, which process comprises adding a tertiaryamino nitrogen-containing acrylic copolymer having a molecular weight of at least 1000' to said polyoxymethylene, contacting the mixture thus obtained With a carboxylic acid anhydride, and thereafter recovering a thermally stable polyoxymethylene carboxylate having said copolymer dispersed therein.

3. A process for the stabilization of a high molecular weight polyoxymethylene having 1 to 2 of its terminal valences satisfied with a hydroxyl group, which process 2 comprises intimately dispersing an acrylic copolymer consisting essentially of 80 parts methyl methacrylate and 20 parts diethylaminoethyl methacrylate upon said polyoxymethylene, and thereafter contacting the resultant mixture with a carboxylic acid anhydride having 4 to 10 carbon atoms, and finally removing unreacted anhydride from said mixture and recovering a thermally stable polyoxymet hylene carboxylate having said acrylic copolymer dispersed therein.

References Cited UNITED STATES PATENTS 2,844,561 7/195=8 Bechtold et a1. 26067 2,993,025 7/1961 Alsup et al 260'42 3,125,551 3/1964 Punderson 260-67 3,218,295 11/1965 Cline 260-67 FOREIGN PATENTS 807,589 1/ 1959 Great Britain.

WILLIAM H. SHORT, Primary Examiner.

L. M. PHYNES, Assistant Examiner. 

3. A PROCESS FOR THE STABILIZATION OF A HIGH MOLECULAR WEIGHT POLYOXYMETHYLENE HAVING 1 TO 2 OF ITS TERMINAL VALENCES SATISFIED WITH A HYDROXYL GROUP, WHICH PROCESS COMPRISES INITIMATELY DISPERSING AN ACRYLIC COPOLYMER CONSISTING ESSENTIALLY OF 80 PARTS METHYL METHACRYLATE AND 20 PARTS DIETHYLAMINOETHYL METHACRYLATE UPON SAID POLYOXYMETHLENE, AND THEREAFTER CONTACTING THE RESULTANT MIXTURE WITH A CARBOXYLIC ACID ANHYDRIDE HAVING 4 TO 10 CARBON ATOMS, AND FINALLY REMOVING UNREACTED ANHYDRIDE FROM SAID MIXTURE AND RECOVERING A THERMALLY STABLE POLYOXYMETHYLENE CARBOXYLATE HAVING SAID ACRYLIC COPOLYMER DISPERSED THEREIN. 